According to accounts from Karen fighters, who have been engaged in a long-running war with Burma's military government, the attack took place just inside the Burmese border, around 16km (10 miles) from the Thai town of Mae Hong Son.
They claim that clouds of yellow vapour began pouring from shells fired at their positions and soon after this many of them felt sick, vomited blood and were unable to walk.
Some later suffered from blisters and acute diarrhoea.
The president of Christian Solidarity Worldwide, Australian physician Dr Martin Panter, has since flown to the area and examined five of the men.
He concluded that their symptoms are synonymous with exposure to some form of chemical attack.
At the moment, the government is pursuing a free trade deal with Myanmar as part of a package deal with ASEAN. I think it's time they stopped.